Optical Transport Fibre and Method for Producing Same

ABSTRACT

In summary, an optical transportation fiber ( 10 ) for transmitting laser beams (S 0 ) comprises at least one fiber core ( 1 ), at least one fiber jacket ( 2, 3 ) and one sheath ( 5 ) encompassing the fiber jacket ( 2, 3 ), wherein an interlayer ( 4 ), the refraction index of which is lower than a refraction index of the corresponding fiber jacket ( 2, 3 ) being in contact with the interlayer ( 4 ), is provided between the fiber jacket ( 2, 3 ) and the sheath ( 5 ). Thereby, the fiber ( 10 ) is formed with at least one outputting means ( 11 ).

The invention relates to an optical fiber according to the preamble of clam 1 and a method for manufacturing an optical fiber and a method for glueing an optical fiber.

Optical fibers are applied as transportation fibers for transmitting light. In particular, the transmission of laser power also with high intensity, e.g. to the machining head during material processing (high power application), is performed by means of such fibers. Thereby, only a portion of the fibers, the fiber core, conducts the light with the desired characteristics, whereas the surrounding layers create the optical framework condition and the mechanical stability of the fiber for its specific application.

However, in particular, when coupling in the light into the fiber and at transition points between fibers, e.g. at plugs, or when splicing, so-called leakage radiation escapes from the fiber core into the surrounding layers, in particular, the so-called cladding. Here, cladding denotes one or several jacket layers of the fiber enclosing the fiber core. From the cladding, which in principle is able to conduct light, the light can transit at e.g. contact spots or by a direct transit into the coating of the fiber. The coating serves in particular for the stability of the fiber and it normally consists of one or several plastic layers e.g. in the form of a buffer directly contacting the fiber and a so-called jacket arranged around the buffer. In the following, the coating is denoted as sheath of the fiber. This sheath can convey the radiation and as the case may be partially absorb it. However, thereby, in particular with high laser power, a high heat build-up of the areas where the radiation escapes, e.g. at a contact spot, can happen. At this locations, the severe heat build-up can lead to a destruction of the fiber. In an extreme case, this can be evenly accompanied with a destruction of the laser source.

Therefore, it is desirable to discharge in controlled manner the leakage radiation from the cladding of the fiber. Therefore, so-called mode strippers are known. Preferably, these are attached at the beginning or at the end of a transportation fiber or at transition points and they can cause a specific outputting of the leakage radiation.

FIG. 1 shows a fiber which is known from the prior art. The fiber 20 has a fiber core 21 which is surrounded by a fiber jacket 22. The fiber jacket 22 is in turn surrounded by a sheath 25 consisting of one or several layers. A mode stripper 26 is formed at the separation layer between the fiber jacket 22 and the sheath 25. In FIG. 1 a, the courses of beams of a first light beam S1 and of a second light beam S2 which do not hit the fiber core (“leakage radiation”) and of a light beam S0 which is transported in an intended manner in the core of the fiber are illustrated. The beam S1 has a flat (large) angle of entry with respect to the perpendicular L on the end face of the fiber, i.e. the angle with respect to a fiber axis (radiation with a high numerical aperture NA), so that this beam can escape from the fiber jacket 22 in the region of the mode stripper 26. The beam S2 has a steeper (smaller) angle of entry (radiation with a low NA) so that this beam is not output at the mode stripper 26 and it can be transported over long distances in the fiber.

As to be seen in FIG. 1 b, a beam entering into the fiber 20, in particular into the cladding, in a flatter angle of entry can be steadily reflected and further transported and escape at a contact spot 29 through the bonding material 28 when a sheath is low refractive and poor in absorbing. Here, the contact point 29 connects a plug 27 via a bonding material 28 which directly contacts the fiber jacket 22 and the fiber 20. Then, the radiation can be absorbed by the bonding material 28 which can lead to a local overheating and, therefore, to a destruction of the fiber 20. Although the problem can be avoided in that an adhesive having a low refraction index and low absorption for the radiation is used; however, such adhesives have an insufficient bonding strength so that their application can be disadvantageous.

In FIG. 1 b, only the leakage radiation emerging when coupling in laser light into the fiber is shown. Leakage radiation can also emerge in the fiber at a splicing or at other contact points. Hereby, the radiation can enter from the core area into the cladding and further spread or invade into the sheath or bonding and lead to a high local heat build-up of the fiber.

In order to avoid heat build-up in local areas, GB 2 379 279 suggests to introduce a layer of lower refractive glass between the cladding and the sheath in order to avoid leakage radiation from entering into the buffer. However, thereby, the leakage radiation is not discharged from the fiber. Thus, the higher or high mode leakage radiation which consequently rests in the cladding of the fiber can reduce the beam quality of the transported beam. This can have negative influences to an application of the laser. In particular, depending on the angle of entry of the radiation into the fiber jacket, the light can also be conducted over long distances. The reduced beam quality is a big problem, in particular with single mode fibers and for applications where a very good beam quality is necessary, e.g. at laser cutting.

With specific fibers, as e.g. fibers having a thick undoped quartz jacket, much leakage radiation can be guided due to the large sectional area.

It is therefore an object of the invention to avoid at least one of the stated disadvantages of the prior art and to provide a fiber which pointedly discharges leakage radiation and avoids an overheating, in particular at contact points or fiber transitions and boundaries.

According to the invention, this object is achieved by the optical fiber according to claim 1 and by means of the method according to claim 9 and of the method for bonding a fiber according to claim 13. Advantageous further developments are subject-matter of the sub-claims.

According to the invention, an optical transportation fiber with at least one fiber core, at least one fiber jacket and one sheath comprises one interlayer which is provided between the fiber jacket and the sheath. By means of such an interlayer, the light transfer between the fiber jacket and the surrounding layers, the sheath or the coating, can be suppressed. This can happen by providing the interlayer having an appropriate thickness as later described in detail.

In particular, this is possible for such fibers, the fiber core of which has a larger refraction index than the jacket material surrounding it as it is the case for most of the fibers. However, it is also conceivable that fibers having a smaller refraction index than the jacket material, e.g. in hollow fibers, are used in alternative embodiments.

It is of course conceivable that the fiber comprises two or more jacket layers arranged around the fiber core.

Thereby, for the interlayer or protective layer, it is valid that their refraction index is smaller than a refraction index of the fiber jacket surrounded by the interlayer. The material of the sheath surrounding the interlayer can have again an arbitrary refraction index, wherein, in a specific case, the material of the sheath can be a bonding material surrounding a contact point of the fiber. The interlayer can in particular be made of a quartz layer of lower refractive quartz.

In a preferred embodiment of the invention, the fiber jacket comprises in particular a refraction index of 1.45, the fiber jacket a refraction index of 1.44 and the interlayer a refraction index of 1.433. Other refraction indexes are of course conceivable in other embodiments.

For the interlayer or the protection layer, it is that its thickness is selected such that it can be removed easily by etching or tarnishing. A thickness of about 5 μm or more, in particular a thickness of about 10 μm, is preferred. Of course, the thickness can be less than 5 μm or more than 10 μm.

Such an optical fiber can further comprise at least one outputting means for outputting radiation, in particular leakage radiation out of the fiber jacket. Therefore, an outputting of radiation out of the fiber jacket which can be adverse for the respective application can be performed at the dedicated locations. This can increase the beam quality. This outputting means can already be jointly formed during the manufacturing process of the fiber.

In general, an outputting means is a means effecting an outputting of leakage radiation out of the fiber along its direction of extension. In one embodiment, the outputting means is a tarnished portion of the fiber manufactured e.g. by sandblasting or etching, wherein in the area of the tarnished portion of the fiber, the sheath and also the interlayer, are removed at least in patches. At this so-formed mode strippers, the light can be spread away out of the fiber jacket. Such outputting means can be provided in regular or irregular distances in the fiber. Preferably, such outputting means are provided after locations where a coupling in of light into a fiber core or, in general, a fiber or before locations where an outputting should happen. This can be the case at the entry of a free beam into the fiber or at a transition between two fibers at a splicing or a mechanical coupling of the fibers or the like.

It is to be mentioned, that it is conceivable that the interlayer also in the area of the outputting means does not have to be completely removed, but it can be merely reduced in thickness and/or removed in sections.

In appropriate embodiments, the fiber can thereby also comprise a second jacket layer which is arranged around the fiber core or around the first fiber jacket. Further suchlike layers are also conceivable. Furthermore, the fiber can be a single-mode fiber or a fiber comprising several cores or which is designed to conduct specific higher modes of the light. Suchlike further types of fibers are adequately known to the expert.

For holding/fixing the fiber in a plug, it is often the case that the fiber comprises a contact spot, wherein an adhesive is arranged adjacent to the interlayer in the area of the contact spot. Normally, the adhesive has a higher refraction index than the interlayer. However, it is also conceivable that the adhesive has a lower refraction index than the interlayer. The interlayer according to the invention can prevent that leakage radiation can escape it the area of the contact spot at the bonding material and that this leads to an undesired heat build-up of the adhesive.

Thereby, an advantage of the invention is that at least a considerably reduced escape of leakage radiation at the contact spot occurs than in the prior art happens. Therefore, no local overheating of the contact sport happens and the risk of a damage of the fiber can be reduced.

With a fiber according to the invention, an outputting of the leakage radiation aimed at these locations where the protection layer is removed is therefore enabled while, otherwise, the leakage radiation is conducted in the fiber.

When providing a protection layer, the refraction index of which is lower than that of the inside adjacent jacket layers, attention should be paid that the electrical field of the leakage radiation transported in the fiber jacket at the boundary to the interlayer always also extends in the direction of the interlayer, i.e. of the medium with the lower refraction index, or intrudes in this medium. There, it loses intensity in the radial direction depending on the optical characteristics. Hereby, it is spoken of an evanescent wave. For avoiding trespassing in the radial direction from the fiber jacket into the outer layers of the fiber, the thickness of the protection layer is to be selected as thick as the field strength of the radiation in the protection layer is accordingly reduced. Therefore, the thickness of the protection layer has to have a multiple of the wave length of the light which is to be transported in the fiber.

Depending on the field of application, the interlayer can preferably comprise a thickness of less than 20 in particular a thickness of 10 μm, 5 μm or less. Such a small thickness for the interlayer can permit to easily remove, in particular to etch away, the interlayer which can enable a better manufacturing compared to fibers known from the prior art.

By providing the interlayer, in a fiber according to the invention, it is no longer necessary that the coating, encompassing the fiber, which can consist e.g. of silicone or acrylate comprises a lower refraction index than the fiber or the fiber jacket. Except that, a material absorbing the radiation can be used for the sheath. Moreover, the low-refractive interlayer arranges that the light can be conducted within the fiber. This can facilitate the dealings with the fiber. In particular, a material which can be selected for the specific application of the fiber can be selected for the encompassing coating as described in the following more in detail.

For manufacturing an optical fiber according to the invention, at least one fiber core, at least one fiber jacket and one sheath arranged around the at least one fiber jacket are formed. Between the fiber jacket and the sheath, an interlayer, the refraction index of which is at least lower than that of the fiber jacket, is formed. When providing an interlayer having a thickness which is larger than the penetration depth of the field into the interlayer, therefore, typically at least considerably larger than the wave length of the light transported in the fiber, the refractive index of the sheath and, therefore, also of its material is at least widely without any influence on the light conducting characteristics of the fiber. Therefore, also other materials than in conventional fibers can be used which can be accompanied by commercial advantages during the manufacturing of the fibers.

In order to enable the connection of fiber ends with other scopes, e.g. the connection of the transportation fiber with a machining head, a plug is usually used. When connecting an optical fiber with a plug, the sheath of the fiber is removed firstly in an area where the connection of the fiber shall be made. After the removal of the sheath, the interlayer forms the outer surface in a radial direction of the fiber. Then, the fiber is brought in contact with its side face with a plug or similar and a bonding material which is consequently in direct contact with the interlayer is arranged around the exposed interlayer. The bonding material can for example be applied by heating or by a so-called shrinking hose. Also, the use of light-curing, in particular UV-light-curing, adhesives is conceivable. The bonding material can also have a larger refraction index than the interlayer and it may be absorbing for the radiation. Thereby, the interlayer prevents that leakage radiation trespasses into the bonding material. Preferably, an outputting element for the leakage radiation is provided before the plug or in the plug. Thereby, an increase of the stability of the contact spot can be achieved because a part of the sheath of the fiber protrudes into the contact spot it is therefore also glued together.

The at least one outputting means in the fiber is preferably formed in a transportation direction of the light through the fiber, therefore, viewed in a direction which is directed along the extension direction of the fiber core along its center axis before a plug or before a contact spot with further components. Therefore a risk of damaging the contact spot, the adhesive and/or the plug or the attached component can be reduced.

In other, alternative embodiments, the fiber can also be formed with more than only one fiber core in order to selectively transport different modes of a plurality of modes of the light. In particular, the fiber can be a single-mode-fiber. Also further types of fibers, e.g. hollow fibers or gradient index fibers, are conceivable in the scope of the invention.

Details, further advantages and developments of the invention are elucidated in detail by means of an embodiment with reference to the drawings.

Thereby:

FIG. 1 shows an optical fiber of the prior art with a) a mode stripper; b) a contact spot;

FIG. 2 shows an optical fiber according to one embodiment of the invention a) in a perspective illustration; b) as refractive index profile;

FIG. 3 shows an illustration of an optical fiber according to one embodiment of the invention with a) a mode stripper, b) a contact spot.

FIG. 2 a) shows an optical fiber 10 according to an embodiment of the invention. The optical fiber 10 has a fiber core 1, an inner fiber jacket 2 and an outer fiber jacket 3. Thereby, the outer fiber jacket 3 corresponds to an overlay layer. As to be seen in FIG. 2 a), the fiber core 1 comprises a circular cross-section and it is arranged centrally in the fiber 10. The inner fiber jacket 2 encompasses circularly the fiber core 1 so that the inner fiber jacket 2 is arranged symmetrically around the center of the fiber 10. The fiber core 1 and the inner fiber jacket 2 are consequently arranged concentrically to another. The outer fiber jacket 3 encompasses also concentrically the inner fiber jacket 2.

In alternative embodiments, the fiber core as well as the inner and/or outer fiber jacket can have a non-circular cross-section, in particular an elliptically, rectangularly or otherwise formed cross-section.

An interlayer 4, also denoted as protection layer, is arranged at the outer surface of the outer fiber jacket 3 in a radial direction outwardly directed from the center of the fiber 10. In an embodiment of the invention, the interlayer 4 has a thickness in the radial direction which enables to etch away this layer. In particular, a thickness of 10 μm, in particular, preferably 5 μm or thinner, is conceivable. Of course, other thicknesses, in particular thicknesses of more than 10 μm are conceivable without departing from the idea of the invention.

A sheath 5 is arranged around the interlayer 4. In the shown embodiment, the sheath 5 also encompasses concentrically the inner components of the fiber 10.

FIG. 2 b) shows a refractive index profile of the fiber from FIG. 2 a). As indicated in FIG. 2 b) by the schematic coordinate system, the refractive index n is thereby plotted depending on a horizontal direction—the radial extension r of the fiber. From FIG. 2 b), it becomes clear that in the shown embodiment, the inner fiber jacket 2 directly encompassing the fiber core 1 has a lower refractive index than the fiber core 1 itself. The refractive index of the interlayer 4 corresponds here to the refractive index of the inner fiber jacket 2. The refractive index of the outer fiber jacket 3 corresponds here to the refractive index of the fiber core 1. It should be noted that in other embodiments, other relationships of the refractive indexes are conceivable. So, the here equal values do not have to be equal but can differ from another as long as the principle characteristics as described before are maintained. In particular, this means only that there is a refractive index drop at the transition between the outer fiber jacket and the interlayer 4.

In the shown embodiment, the sheath 5 consists of two layers, the buffer 5 a and the jacket 5 b. In this embodiment, the buffer 5 a has a lower refraction index than the interlayer 4 so that the leakage radiation with high NA is also held in the fiber by the total reflection. The jacket 5 b of the fiber has no optical function here but it only serves for the mechanical stability. With an appropriate thickness of the interlayer 4, the sheath 5 can also attain any other refraction index without impairing the function of the fiber. In the following, it is only spoken of the sheath 5 due to convenience. However, it is to be noted that the sheath 5 can be formed as described above.

FIG. 3 shows a fiber according to the invention with the interlayer 4 between the outer fiber jacket 3 and the sheath 5. For reasons of clarity, the inner fiber jacket 2 is not illustrated here. However, in alternative embodiments, the inner or the outer fiber jacket 2, 3 can also be omitted. In FIG. 3 a, the beam courses within the fiber jacket 3 are illustrated for two different mis-coupled-in beams S1, S2 (“leakage radiation”) and the radiation S0 being coupled in into the core. A first light beam Si is shown with a flat angle of entry with respect to a perpendicular to the fiber or the fiber jacket 2, 3 (high NA). Consequently, this beam S1 hits in an acute angle on the longitudinal side within the fiber 10, therefore, on a side extending parallel to the fiber core 1 and, therefore, it has a high quantity of reflection points within the fiber 10. Thus, the beam S1 most likely hits on an outputting means provided in the fiber 10. In the following, the outputting means is also denoted as mode stripper 11.

For example, the mode stripper 11 can be manufactured by etching away the protection layer 4 and tarnishing the surface of the fiber jacket 3 and illustrates an area with inhomogeneous refraction characteristics and irregular surface in the shown embodiment. At the mode stripper 11, the beam can couple out of the fiber jacket 3. In alternative embodiments, it is also conceivable that the outputting means 11 is manufactured in another manner, e.g. by mechanical roughening of the interlayer, applying a higher refractive material, or the like.

A second beam S2 shown in FIG. 3 a intrudes into the fiber 10 in an angle which has, to the fiber end, an acute angle with respect to the perpendicular L (low NA) and, consequently, it has a lower quantity of reflection points in the course of the further propagating in the fiber jacket 2, 3 which reduces the likelihood that the beam S2 is outputted at one of the mode stripper 11.

As furthermore shown in FIG. 3 a, the mode stripper 11 is formed such that it acts at a location without any sheath. In the shown embodiment, the mode stripper has been formed by etching away the interlayer 4 of the fiber 10. It is not be noted that also a merely partial etching away of the interlayer 4 can be sufficient.

FIG. 3 b shows a fiber according to the invention which has been connected to a plug in the area of a contact spot 9.

A plug 7 accommodating the end of the fiber is arranged around the contact spot 9. In the plug 7, a bonding material 8 directly contacting the fiber, in particular the outer interlayer 4 when bonding, is provided. Together with the plug 7, the bonding material 8 extends in a longitudinal direction of the fiber 10, therefore, in a direction perpendicular to the radial direction of the fiber 10, and forms the contact spot 9. In this manner, the fiber can be connected also to other components.

As shown in FIG. 3 b, by forming the fiber with the interlayer 4, a beam running in the fiber jacket 3 is prevented from entering into the sheath 5. Even at contact spots where the adhesive normally has a high refractive index, a trespassing is prevented by providing the interlayer 4 between the fiber jacket 3 with a high refraction index and the bonding material 8 normally also having a high refraction index.

By a sufficiently large dimensioning in a radial direction of the interlayer 4, a trespassing out of the fiber jacket into the sheath can completely be prevented.

Thereby, the interlayer must not have any homogenous characteristics for example in view of its thickness or its refraction index.

In summary, an optical transportation fiber 10 for transmitting laser beams S0 comprises at least one fiber core 1, at least one fiber jacket 2, 3 and one sheath 5 encompassing the fiber jacket 2, 3, wherein an interlayer 4, the refraction index of which is lower than a refraction index of the corresponding fiber jacket 2 or 3 being in contact with the interlayer 4, is provided between the fiber jacket 2, 3 and the sheath 5. The interlayer prevents an escape of leakage radiation S1, S2 out of the fiber jacket into the sheath 5 or into a contact spot 9. Thereby, the fiber 10 is formed with at least one outputting means 11. 

1. Optical fiber for transporting laser radiation with at least one fiber core, at least one fiber jacket and one sheath encompassing the fiber jacket, wherein a interlayer is provided between the fiber jacket and the sheath, wherein a refraction index of the interlayer is lower than a refraction index of the fiber jacket, and at least one outputting means for outputting radiation out of the fiber jacket is provided in the fiber, characterized in that the fiber comprises a contact spot, wherein an adhesive is arranged in the area of the contact spot adjacent to the interlayer.
 2. Optical fiber according to claim 1, characterized in that the outputting means is a tarnished portion of the fiber, wherein the interlayer is removed in the area of the tarnished portion of the fiber.
 3. Optical fiber according to claim 1, wherein a thickness of the interlayer is selected such that the interlayer is removable by an etching process, wherein the thickness of the interlayer is less than 20 μm.
 4. Optical fiber according to claim 3, wherein the thickness of the interlayer is in a range between 5-10 μm.
 5. Optical fiber according to claim 1 characterized in that the fiber comprises a further jacket layer which is in contact with the fiber jacket and arranged around it.
 6. Optical fiber according to claim 1, characterized in that the fiber (10) is a single-mode fiber.
 7. Optical fiber according to claim 1, characterized in that the fiber comprises a plurality of fiber cores.
 8. Optical fiber according to claim 1, characterized in that the outputting means is provided before the contact spot in an expansion direction of the light along the fiber.
 9. Method for manufacturing an optical fiber for transporting laser radiation, comprising the steps: forming a fiber core, forming at least one fiber jacket which is arranged around the fiber core, and forming a sheath arranged around the fiber jacket, wherein the fiber is formed at least in sections with an interlayer between the fiber jacket and the sheath, and wherein the interlayer is formed with a refraction index which is lower than a refraction index of the fiber jacket, characterized in that the fiber is formed with an interlayer at least in the area of the contact spot.
 10. Method according to claim 9, characterized in that the fiber is formed with an outputting means for leakage radiation.
 11. Method according to claim 10, wherein the outputting means is formed by etching away the interlayer.
 12. Method for bonding an optical fiber according to claim 9, characterized in that the sheath of the fiber is removed in the area of a contact point in such a manner that the interlayer forms a surface of the fiber, and a bonding material is arranged around the interlayer and it is brought in contact with the interlayer.
 13. Method for bonding an optical fiber according to claim 12, wherein the sheath is removed in such a manner that, during bonding, the sheath protrudes into the contact point. 